sandbox: add missing apt-get purge
sandbox: add missing apt-get purge

Change-Id: I0e89524ca9ed6b73ef478c32fdb6da7a11cafe76
Reviewed-on: https://go-review.googlesource.com/27319
Reviewed-by: Jessica Frazelle <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@jessfraz.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/playground,golang/playground,golang/playground